Thunder Group of Engineering Institute Makes Important Progress in Polymer Thermoelectric Materials Research

[ Instrument network instrument research and development ] Polymer materials have broad application prospects in the fields of optoelectronics, energy conversion and energy storage devices due to their low toxicity, light weight, large area solution processing and flexibility. Among them, organic thermoelectric materials have attracted widespread attention as an energy conversion material that can directly convert thermal energy and electrical energy into each other.

Obtaining highly efficient and equivalent p-type and n-type thermoelectric materials is a necessary condition for constructing high-efficiency thermoelectric devices. In the field of organic thermoelectric materials, p-type conjugated polymers (such as PEDOT) have shown a high thermoelectric figure of merit (ZT> 0.4) close to that of inorganic thermoelectric materials, which is mainly due to the high conductivity (> 1000Scm) of such materials -1) and high power factor (> 300μWm-1K-2). In contrast, only a few n-type conjugated polymers have an electrical conductivity close to or slightly above 1 Scm-1, and the power factor is generally lower than 10 μWm-1K-2.

In recent years, with the emergence of various structural units constituting conjugated polymers with excellent properties, the carrier mobility of DA-type conjugated polymers has been significantly improved. Among them, pyrrolopyrrole dione (DPP) is a type of structural unit that has received wide attention. Currently, DPP-based DA conjugated polymers reported in the literature have shown electron mobility in excess of 5 cm 2 V-1 s-1. The value is even better than the hole mobility of current high-performance p-type conjugated polymers (such as PEDOT, PBTTT, etc., which is about 1 cm2 V-1 s-1). However, the conductivity of n-doped DPP conjugated polymers is usually low (0.1 ~ 1Scm-1), which is mainly caused by the lower n-type doping efficiency of such polymers.

Figure 1. Conformation-locked conjugated polymer P (PzDPP-CT2)

Based on the above research background, the Thunder Research Group of the School of Engineering of Peking University designed and synthesized a new type of pyrazinylpyrrolopyrrole dione (PzDPP) structural unit. Among the DPP structural units reported in the literature, PzDPP has the lowest LUMO energy level. A new DA-type conjugated polymer P (PzDPP-CT2) was obtained by polymerizing PzDPP and the electron-deficient 3,3'-dicyano-2,2'-bidithiophene structural unit. The polymer exhibits a conformationally-locked coplanar skeleton structure and a LUMO energy level as low as −4.03 eV. A low LUMO energy level is beneficial to improve the n-type doping efficiency of the polymer. After being doped with n-type dopants, the polymer P (PzDPP-CT2) showed a conductivity as high as 8.4 Scm-1 and a power factor of 57.3 μWm-1K-2. The electrical conductivity is significantly better than the electrical conductivity of n-type doped conjugated polymers reported in the literature, and the power factor is also a higher value of the currently processable n-type polymer thermoelectric materials.

Figure 2. Schematic of a flexible thermoelectric device based on polymer P (PzDPP-CT2)